This invention relates to preparation of chromium-containing olefin polymerization catalysts.
Supported chromium oxide catalyst can be used to prepare olefin polymers in a hydrocarbon solution or diluent to give a product having excellent characteristics from many standpoints.
The chromium can be incorporated into the support in several different ways which are well known in the art. One such method involves coprecipitating a chromium compound with the support, for instance, by including a chromium compound in a silicate and thereafter combining same with an acid to form a gel. Frequently, the chromium compound is introduced by means of an aqueous impregnation of the support followed by removal of the water. Because water has an effect on the pore structure and/or surface of silica which is generally adverse, it is sometimes desirable to incorporate the chromium compound by means of a hydrocarbon solution of a soluble zerovalent chromium compound such as dicumene chromium. Finally, the chromium compound can simply be physically mixed with the support, for instance by dry milling the support and the chromium compound.
Each of these means for incorporating chromium results in a catalyst having different characteristics even though after the activation the chromium may be oxidized to the same chromium oxide form. This is because the pore structure and/or surface of the support may be affected by the manner of chromium addition. Particularly, in the aqueous impregnation of chromium, the pore structure of the support may be damaged by the water, particularly by the removal of the water. In addition, water, primary alcohols and other materials which easily rehydrate the silica surface can have an adverse effect on the resulting catalyst and thus are not suitable solvents. However, this cannot be overcome simply by utilizing a hydrocarbon or other nonaqueous solution since ionic chromium compounds are generally insoluble in the type of nonaqueous solvents which do not easily rehydrate the silica surface. The use of an alcohol to dissolve a chromium compound such as CrO.sub.3 is not always desirable either for the reason that the OH groups of the solvent may react with the silica or the solvent reacts with the chromium compound.